Clock generation is an important part of any electronic system. In integrated circuit devices, a crystal oscillator may provide excellent stability across a number of operating variables related to integrated circuit technology such as fabrication misalignment, voltage supply variations, and temperature variation (known as PVT variations in the industry). The most common material used for such a crystal oscillator is quartz. But, it is typically not possible to integrate a crystal oscillator onto an integrated circuit chip using a conventional CMOS process as such a step is not part of the IC processing, and therefore, must then be integrated into a related board. Further, requiring a customer to install a quartz crystal outside of the integrated circuit chip may increase the footprint that the chip package occupies, and, thus, may render the integrated circuit chip unsuitable for applications in which circuit-board area is at a premium.
To overcome the need for a quartz crystal in a crystal oscillator, a CMOS ring oscillator is a conventional solution that provides a ready-made solution to integrated circuit clocking. However, the output frequency of a CMOS ring oscillator does not remain constant over PVT variations. That is, PVT variations cause undesired frequency fluctuations in the clock signal generated and, therefore, are problematic for application requiring a more stable clock signal.